The invention relates to a process for preparing impact-modified thermoplastic molding compositions which comprise a soft phase made from a rubber dispersed in a hard matrix composed of vinylaromatic monomers.
There are various known continuous and batch processes, in solution or suspension, for preparing impact-modified polystyrene. In these processes a rubber, usually polybutadiene, is dissolved in monomeric styrene, which is polymerized in a preliminary reaction to a conversion of about 30%. The formation of polystyrene and the associated depletion of monomeric styrene results in a change in the phase coherence. During this process, known as phase inversion, grafting reactions also occur on the polybutadiene and these, together with the intensity of agitation and the viscosity, affect the formulation of the disperse soft phase. The polystyrene matrix is built up in the main polymerization which follows. Processes of this type, carried out in various types of reactor, are described, for example, in A. Echte, Handbuch der technischen Polymerchemie, VCH Verlagsgesellschaft Weinheim, Germany, 1993, pages 484-489 and in U.S. Pat. Nos. 2,727,884 and 3,903,202.
These processes require complicated comminution and dissolving of the separately prepared rubber, and the resultant polybutadiene rubber solution in styrene has to be filtered before the polymerization to remove gel particles.
The required solution of rubber in styrene may also be prepared by anionic polymerization of butadiene or butadiene/styrene in nonpolar solvents, such as cyclohexane or ethylbenzene, followed by addition of styrene (GB 1 013 205 and EP-A-0 334 715) or by incomplete conversion of butadiene in styrene (EP-A 0 059 231 and EP-A 0 304 088) followed by removal of the unconverted butadiene. The rubber solution is then subjected to a free-radical polymerization.
Processes for preparing thermoplastic molding compositions by anionic polymerization of styrene in the presence of a rubber are known, for example, from DE-A-42 35 978 and U.S. Pat. No. 4,153,647. The resultant impact-modified products have lower contents of residual monomers and oligomers, compared with the products obtained via free-radical polymerization.
Anionic polymerization of styrene proceeds very rapidly and gives very high conversions. The high polymerization rate and the heat generation associated with this mean that on an industrial scale these processes are restricted to very dilute solutions, low conversions or low temperatures.
Alkyl compounds of alkaline earth metals, of zinc and of aluminum have therefore been described as retardant additives for anionic polymerization of styrene (WO 97/33923 and WO 98/07765) or butadiene in styrene (WO 98/07766). Controlled anionic polymerization of styrene and butadiene to give homopolymers or styrene-butadiene copolymers is possible with these additives.
WO 98/07766 moreover describes the continuous preparation of impact-modified molding compositions using the styrene-butadiene rubbers which can be obtained by means of the retardant additives in styrenic solution. However, the rubbers obtainable by this process always comprise small amounts of copolymerized styrene in the butadiene blocks.
It is an object of the invention to avoid the disadvantages mentioned and to develop a process which permits the preparation of impact-modified molding compositions which are low in residual monomers and in oligomers. The process should furthermore ensure simple and reliable control of the reaction. It should be suitable for using a very large number of types of rubber, in order to permit a wide range of properties in the impact-modified molding compositions.
Another object was a continuous process for anionic polymerization of impact-modified molding compositions with simple and reliable control of the reaction.
We have found that this object is achieved by means of a process for preparing impact-modified thermoplastic molding compositions which comprise a soft phase made from a rubber dispersed in a hard matrix composed of vinylaromatic monomers, where the hard matrix is prepared by anionic polymerization in the presence of a metal organyl compound of an element of the second or third main group, or of the second subgroup, of the Periodic Table.
Metal organyl compounds of an element of the second or third main group, or of the second subgroup, of the Periodic Table which may be used are the organyl compounds of the elements Be, Mg, Ca, Sr, Ba, B, Al, Ga, In, Tl, Zn, Cd, Hg. These metal organyl compounds are also termed retarders, due to their effect during anionic polymerization. Preference is given to the magnesium and aluminum organyl compounds. For the purposes of the invention, organyl compounds are the organometallic compounds of the elements mentioned with at least one metal-carbon a bond, in particular the alkyl or aryl compounds. The metal organyl compounds may also contain, on the metal, hydrogen, halogen, or organic radicals bonded via heteroatoms, giving compounds, such as alcoholates or phenolates. The latter are obtained, for example, by complete or partial hydrolysis, alcoholysis or aminolysis. It is also possible to use mixtures of different metal organyl compounds.
Suitable magnesium organyl compounds have the formula R2Mg, where R, independently of one another, are hydrogen, halogen, C1-C20-alkyl or C6-C20-aryl. Preference is given to dialkylmagnesium compounds, in particular the ethyl, propyl, butyl or octyl compounds which are commercially available products. Particular preference is given to (n-butyl)(sec-butyl)magnesium, which is soluble in hydrocarbons.
Aluminum organyl compounds of the formula R3Al may be used, where R, independently of one another, are hydrogen, halogen, C1-C20-alkyl or C6-C20-aryl. Preferred aluminum organyl compounds are the aluminum trialkyl compounds, such as triethylaluminum, triisobutylaluminum, tri-n-butylaluminum, triisopropylaluminum and tri-n-hexylaluminum. Particular preference is given to triisobutylaluminum. Use may also be made of aluminum organyl compounds produced by partial or complete hydrolysis, alcoholysis, aminolysis or oxidation of aluminum alkyl compounds or of aluminum aryl compounds. Examples of these are diethylaluminum ethoxide, diisobutylaluminum ethoxide, diisobutyl(2,6-di-tert-butyl-4-methylphenoxy)aluminum (CAS No. 56252-56-3), methylaluminoxane, isobutylated methylaluminoxane, isobutylaluminoxane, tetraisobutyldialuminoxane and bis(diisobutyl)aluminum oxide.
The retarders described generally do not act as polymerization initiators. The anionic polymerization initiators used are usually mono-, bi- or polyfunctional alkali metal alkyl compounds, alkali metal aryl compounds or alkali metal aralkyl compounds. It is useful to use organolithium compounds, such as ethyl-, propyl-, isopropyl-, n-butyl-, sec-butyl-, tert-butyl-, phenyl-, diphenylhexyl-, hexamethylenedi-, butadienyl-, isoprenyl- or polystyryllithium, or the polyfunctional compounds 1,4-dilithiobutane, 1,4-dilithio-2-butene or 1,4-dilithiobenzene. The amount of alkali metal organyl compound required depends on the desired molecular weight and on the type and amount of the other metal organyl compounds used, and also on the polymerization temperature. It is generally in the range from 0.002 to 5 mol percent, based on the total amount of monomers.
Preferred vinylaromatic monomers for the hard matrix are styrene, xcex1-methylstyrene, p-methylstyrene, ethylstyrene, tert-butylstyrene, vinyltoluene and 1,1-diphenylethylene, or mixtures. Styrene is particularly preferred.
The rubber used for the soft phase may be any desired diene rubber or acrylate rubber, or mixtures which have a certain compatibility with the vinylaromatic hard matrix. It is therefore advantageous if the rubber comprises a certain proportion of styrene blocks, since the anionic polymerization of the hard matrix does not produce any compatibility of the rubber via grafting of monomers which form the hard matrix.
The rubber used is preferably a styrene-butadiene block copolymer or a mixture of a styrene-butadiene block copolymer with a homopolybutadiene, where the styrene content, based on the entirety of the rubber, is in the range from 5 to 50% by weight, preferably from 10 to 45% by weight, particularly preferably from 20 to 40% by weight. The content of residual butadiene in the rubber should be less than 200 ppm, preferably less than 100 ppm, in particular less than 50 ppm.
In a preferred version of the process the rubber solution is prepared in a first step by the usual methods of anionic polymerization and styrene is used for dilution. In a second step without further addition of solvents the hard matrix is polymerized with phase inversion to a conversion of at least 90%, based on the hard matrix.
It is useful to polymerize the rubber in an aliphatic, isocyclic or aromatic hydrocarbon or hydrocarbon mixture, preferably in benzene, toluene, ethylbenzene, xylene, cumene or cyclohexane. Toluene and ethylbenzene are particularly preferred. The polymerization of the rubber may also be carried out in the presence of liquid additives. These are usually not added until during or after the polymerization of the hard matrix. The rubber may, for example, be prepared in mineral oil or in a mixture of mineral oil and the abovementioned hydrocarbons. This makes it possible to reduce the viscosity or the amount of solvent.
A very high solids content is selected for the resultant solution. Its upper limit is principally determined by the viscosity of the solution. When a styrene-butadiene rubber is used, the viscosity, and therefore the possible solids content, depends inter alia on the block structure and the content of styrene. It is useful to select a solids content in the range from 15 to 50% by weight, preferably from 20 to 40% by weight.
The polymerization of the rubber may be carried out continuously or batchwise with a buffer tank. Continuous preparation may be carried out in continuous stirred tank reactors (CSTRs), such as stirred-tank reactors (or stirred-reactor cascades) or in circulating reactors or plug-flow reactors (PFRs), such as tubular reactors with or without internals, or in combinations of various reactors. Batchwise preparation is preferably carried out in a stirred-tank reactor.
The rubbers may be polymerized in the presence of a polyfunctional alkali metal organyl compound, or linked to give a star shape during or after the polymerization using a polyfunctional coupling agent, such as polyfunctional aldehydes, ketones, esters, anhydrides or epoxides. Symmetrical or asymmetrical star block copolymers can be obtained here by coupling of identical or different blocks.
After completion of the polymerization, the living polymer chains may be closed off with a chain terminator instead of a coupling procedure. Suitable chain terminators are protonating substances or Lewis acids, such as water, alcohols, aliphatic or aromatic carboxylic acids, or also inorganic acids, such as carbonic or boric acid. The amount of chain terminator added is in proportion to the amount of living chains.
It is useful to dilute the solution directly after the end of the reaction with the vinylaromatic monomer, in order to make subsequent handling easier.
The resultant rubber solution is polymerized as described above in a second step, if desired with addition of further vinylaromatic monomer.
The conversion, based on the vinylaromatic monomer of the hard matrix, is generally greater than 90%. The process may in principle also give complete conversion.
The content of rubber, based on the entire molding composition, is usefully from 5 to 25% by weight. It essentially depends on the type of rubber used and on the desired properties of the impact-modified molding composition.
The solids content achieved at the end of the reaction in the second step is generally in the range from 70 to 90%, in particular from 75 to 85%, for the abovementioned ranges of solids content of the rubber solution and the usual rubber content in the molding composition.
Surprisingly, it has been found that the polymerization of the hard matrix can be carried out without further addition of anionic polymerization initiator if use is made of a rubber solution which, as described above, has been prepared by anionic polymerization and terminated by chain termination or coupling. In this case, the metal alkyl compounds, which otherwise have only a retarding effect, can initiate the polymerization of the hard matrix. This results in simpler metering and control than when using an initiator/retarder mixture.
The anionic polymerization of the hard matrix in the second reaction zone is preferably initiated exclusively by addition of a magnesium dialkyl compound. Preference is given to a magnesium dialkyl compound which contains at least one secondary or tertiary alkyl group. (N-butyl)(s-butyl)magnesium is very particularly preferred.
The polymerization of the rubber and of the hard matrix may be carried out batchwise or continuously in stirred-tank reactors, circulating reactors, tubular reactors, tower reactors or rotating-disk reactors, as described in WO 97/07766.
The resultant molding compositions may be freed from solvents and from residual monomers in a conventional manner, by using devolatilizers or vented extruders at atmospheric pressure or reduced pressure and at temperatures of from 190 to 320xc2x0 C. The solvent removed may be reintroduced to the rubber synthesis if desired after a purification step. To avoid accumulation of contaminants, a relatively small amount of the solvent may be removed from the process and used at another time.
The resultant product has a content of less than 200 ppm of residual monomers, preferably less than 100 ppm, in particular less than 50 ppm.
It can be useful to crosslink the rubber particles, by controlling the temperature appropriately and/or by adding peroxides, in particular those with a high decomposition temperature, such as dicumyl peroxide.